User Varialble input failure
You have defined a RNA property, it will be available on any instance of this class( all scenes will have this property ) , to access this property use the following line  :

XX = context.scene.XX
# in the UI
row.prop(context.scene, 'XX')


you can also define an ID property which will be available only on this scene :

context.scene['XX'] = 10.0
print(context.scene['XX'])
#in UI
row.prop(context.scene, '["XX"]')


To learn more about this Topic refer to this DOC page : RNA properties versus ID properties